A valve actuation mechanism of an internal combustion engine is in general liable to wear and/or thermal distortion that change valve gaps of the valve actuation mechanism in operation. To properly compensate for such change in valve gap, a hydraulic lash adjuster may be used.
FIG. 19 shows a conventional hydraulic lash adjuster, which has a body 2 (hereinafter referred to as adjuster body 2) mounted in a mounting bore 30 formed in a cylinder head 10 of the engine. The adjuster body 2 comprises a body 24 fitted in the mounting bore 30 and a plunger 26 mounted in the body 24 so that the plunger is slidable in the upward and downward directions. There is formed in the plunger 26 a reservoir 28, which communicates with an oil gallery 32 that faces the mounting bore 30 through a small hole 24b, formed in the body 24. The reservoir 28 also communicates with a high pressure chamber 29 via a small hole 27b formed in the reservoir 28. During operation, the reservoir 28 and the high pressure chamber 29 are filled with hydraulic oil supplied from the oil gallery 32. Reference numerals 14, 16 and 17 respectively indicate a valve member, a cam, and a rocker arm which together constitute the valve actuation mechanism. When a pressure is applied to the hydraulic oil, a check ball 25a disposed in the high pressure chamber 29 closes the small hole 27b to lock the plunger 26 with its top end adapted to serve as a fulcrum of the rocker arm 17. As the cam nose 16a of the cam 16 pushes the rocker arm 17 downward, the rocker arm 17 in turn pushes down the valve member 14 against the force of a spring 15, and opens the air passage 12 associated therewith. As the cam 16 rotates, the valve member 14 is returned to its close position by the action of the spring 15. A plunger spring 23 keeps the plunger 26 in abutting contact with the rocker arm 17 at all times, thereby compensating for the gap between them caused by thermal deformations that occur in the valve actuation mechanism during operation.
Installed in the reservoir 28 is a cylindrical body 6, which partitions the reservoir 28 into an inner chamber 28a communicated with a high pressure chamber 29 and an outer chamber 28b communicated with a small hole 27a serving as an oil supply hole. It is noted that during operation the oil level in the inner chamber 28a is kept up to the level H1 in the reservoir communicated with the high pressure chamber 29 as shown in the FIG. 19 even when the adjuster body 2 is installed inclined, so that the reservoir can store a large amount of oil therein and adequately prevent air from being induced from the reservoir into the high pressure chamber 29 when the engine is stopped once and restarted.
That is, were it not for the inner chamber 28a, the oil level in the reservoir 28 would become as low as level H2 (FIG. 19) of the oil supply passage 27a when the engine is stopped. Hence, at the time of restarting the engine, the air staying above the oil surface would be sucked into the high pressure chamber 29 together with the hydraulic oil sucked from the reservoir 28 into the high pressure chamber 29. Particularly, when the engine is stopped with the cam nose 16a sitting on the rocker arm 17, the plunger 26 is held at the lowest (bottom) position. If the engine is restarted under such condition, the plunger 26 undergoes a maximum possible stroke in the body 24, so that the maximum amount of the hydraulic oil could be drawn into the high pressure chamber 29. In actuality, however, since no hydraulic oil is supplied from the engine to the reservoir 28 while the engine is stopped, it is almost impossible then to secure a sufficient amount of oil in the reservoir 28. As a consequence, a violent air suction phenomenon takes place when the engine is restarted. Should air be drawn in the high pressure chamber 29, the hardness of the hydraulic oil in the high pressure chamber 29 is extremely reduced (so that the hydraulic oil becomes a sponge-like material) when the plunger 26 is pressed down. This disenables proper compensation of the valve gap by the plunger. In contrast, in the valve actuation mechanism shown in FIG. 19, a large amount of oil is secured (as high as oil level H1) in the inner chamber 28a of the reservoir 28 when the engine is stopped that air suction into the high pressure chamber 29 at the time of restarting the engine is prevented.